Optical and Electrical Characterization

Chapter 3 Plasmon-induced Hot Electron Devices with Plasmonic Absorbers

3.3 Optical and Electrical Characterization

The optical absorption spectrum of the device was performed by measuring the reflection R and transmission T in an infrared microscope coupled to a grating spectrometer. The reflection spectra were normalized to a silver mirror and the transmission spectra were normalized to transmission through air and absorption was calculated as A = 1 - T - R. The optical absorption of these devices is shown in Fig. 3.2c.

The devices show broadband optical absorption in the near IR region with a maximum value of 95% and a FWHM of 400nm. Furthermore, it is evident that the absorption and

photoresponsivity spectrum can be readily varied by tuning the dimensions of the structure.

It should be noted that, although in the proposed structure the upper and lower metal constituents are electrically isolated, in the fabricated structure there is a thin coating of metal on the sidewall of the Si ridge due to the slight sidewall angle. As a result, the absorption peak positions of the fabricated structure are slightly blue shifted from the proposed structure without sidewall. However, good agreements were still observed between the simulated absorption spectrum with sidewall coating and the measured absorption spectrum (Fig. 3.2c). The connection between the upper and lower antennae also ensures both of the metal layers are included in the electrical circuit and can contribute to the total photocurrent.

To characterize the hot-electron transfer efficiency of the devices, the sample was wire bonded to a chip carrier and the photocurrent measurements were performed on the MPA detectors across a wavelength range of 1200nm to 1500nm (Fig. 3.2d). These MPA detectors show a broad photoresponsivity spectrum with a peak value of 3.37mA/W, 3.05mA/W and 2.75mA/W for devices D1, D2 and D3 without external bias, respectively. Furthermore, the current varies linearly with the incident power, indicating that non-linear processes are not playing a role in the response. These photoresponsivities are several orders of magnitude larger than previously demonstrated LSPR-based nanoantenna photodetectors[46] and over four times larger than the SPP-based metal grating photodetectors[48]. The photoresponsivities of the MPA detectors are comparable with the recently reported deep trench cavity structure[125] at short wavelengths.

However, the MPA detectors show a broadband response with much larger

photoresponsivities at longer wavelengths.

Figure 3.3. (a). Schematic of the hot electron transfer process over the Schottky barrier formed by the metal-semiconductor interface. Steps 1 to 3 correspond to hot electron generation, diffusion to the Schottky interface, and transmission to the conduction band of semiconductor. (b) FDTD simulation of absorbed power density in a 1D MPA device at 1250nm, 1350nm and 1500nm (plotted on a logarithmic scale for better visualization).

(c) Calculated photoresponsivity (solid lines) and absorption (dash lines) in a MPA device. The total (red), upper resonator (green) and lower resonator (blue) contributions are plotted separately.

The photoresponsivity of the MPA detectors can be understood by considering the hot electron transfer process over the Schottky barrier which is dictated by a three-step model[58], [59] as shown in Fig. 3.3a: (1) Plasmons non-radiatively decay into hot electrons (optical absorption), (2) hot electrons transport to the metal/semiconductor interface before they thermalize, and (3) hot electrons inject into the conduction band of the semiconductor through internal photoemission. Here, we analyze the two different components in the MPA detectors separately, the upper antenna and the lower antenna, which yield different hot electron generation and quantum transmission probabilities. As a result, the photoresponsivity spectrum of MPA detectors can be calculated as:

𝑅(𝑣) = ∑_{𝑖=𝑢,𝑙}𝐴₁^𝑖(𝑣)𝜂₂(𝑣)𝜂₃^𝑖(𝑣) (3.1)

where u and l represent the upper and lower antenna, respectively, and ν is frequency. 𝐴₁^𝑖(𝑣) is the optical absorptivity of each component, which can be determined by the measured total absorption spectrum of the MPA detectors along with the absorption distribution in each antenna layer. The absorption distribution in the MPA devices (Fig 3.4b) were calculated numerically using the local ohmic loss, given by,

𝑄(𝑟, 𝜔) =¹₂𝜔Im(𝜀_𝑚)|𝐸⃑ (𝑟, 𝜔)|² (3.2) where |𝐸⃑ (𝑟, 𝜔)| is the local electric field and Im(𝜀_𝑚) is the imaginary part of the metal permittivity. This allows us to determine the wavelength dependent absorptivity of the upper, 𝐴₁^𝑢, and lower, 𝐴₁^𝑙, antenna (dash lines in Fig. 3.3c). 𝜂₂(𝑣) is the probability that the hot electron will transport to the Schottky interface and is dependent on the spatial distribution and mean free path of the hot electrons[128]. 𝜂₃^𝑖(𝑣) is the internal photoemission of hot electrons across the Schottky interface for the upper and lower antenna and is calculated using the modified Fowler equation[57], [60],

𝜂₃^𝑖(𝑣) = 𝐶_𝐹^{𝑖 (ℎ𝑣−𝑞𝜙}_ℎ𝑣 ^𝐵)2 (3.3) where 𝐶_𝐹^𝑖 is the device-specific Fowler emission coefficient, ℎ𝑣 is the photon energy, and 𝑞𝜙_𝐵= 0.54𝑒𝑉 is the Schottky barrier height for the MPA devices which is extracted from the current-voltage characteristics of the device[60]. Fitting the experimentally measured photoresponsivity spectrum with equation (3.1), where 𝜂₂(𝑣) and 𝐶_𝐹^𝑖 are treated as fitting parameters, results in good agreement with the experimental data as shown in Fig. 3.2d. The extracted ratio of 𝐶_𝐹^𝑙⁄𝐶_𝐹^𝑢 is around 3 for all three devices, indicating that hot electrons generated in the lower antenna have a higher quantum transmission probability compared with the upper antenna. This effect is mainly due to

the formation of a 3D Schottky interface between the semiconductor and the embedded bottom antenna, allowing the hot electrons to cross the interface over a larger emission cone[123]. While total absorption drops at longer wavelengths, the bottom antenna layer’s contribution remains relatively constant, resulting in photocurrent being dominated by the bottom layer at long wavelengths (Fig. 3.3c). Strong broadband absorption 𝐴₁(𝑣) and high transport efficiency 𝜂₂(𝑣) in the ultrathin metal film ultimately ensure high photoresponsivity over a large bandwidth.